A photodetector can be a monolithic semiconductor structure or a heterostructure consisting of a photocathode and an electron sensor. The electron sensor in the heterostructure photodetector can be an electron-bombarded charge coupled device (EBCCD) or a micro-channel plate (MCP). The photocathode and the electron sensor generally are mutually facing planar devices separated by a vacuum gap across which a large (e.g., 10 kV) electric field is imposed. As photons from a field of view strike the photocathode, electrons are emitted from the surface of the photocathode facing the electron sensor. The emitted electrons are accelerated across the vacuum gap and strike the electron sensor. The electron sensor amplifies the electron current. In the case of the EBCCD, amplification of the electron current is obtained by exploiting the quantum yield of the semiconductor material of the EBCCD. In the case of the MCP, amplification is obtained by providing the interior surfaces of the glass tubes constituting the MCP with a high electron yield surface. Each incoming electron ricochets on a tube interior surface many times, producing as many as 300 additional electrons for each incoming electron.
The performance of the photodetector is limited by the efficiency with which the photocathode emits electrons in response to incoming photons. The photocathode is generally a planar semiconductor crystal. Each incident photon creates a hole-electron pair in the semiconductor crystal by elevating an electron from the valence band to the conduction band, leaving a hole in the valence band. Generally, a semiconductor material having a bandgap energy corresponding to the infrared region (such as GaAs) does not readily emit electrons from its surface when struck by photons, due to an energy barrier that arises at the crystal surface of the semiconductor. In order for surface emission to occur, the electron must overcome both the work function of the surface and the band gap energy of the semiconductor. Conventionally, this problem is overcome by “activating” the surface of the photocathode in such a manner that this energy barrier is overcome. In the case of a UV or IR photodetector, the photocathode can be a group III semiconductor or group III-V compound semiconductor, and the “activation” consists of depositing a thin Cesium (Cs) coating on the crystal surface. The Fermi levels in the Cs and semiconductor layers equilibrate at the interface between the layers, forcing the valence and conduction band structures in the semiconductor layer to “bend” so much that the conduction band at the surface is below the Fermi level and the bulk conduction band bottom lies above the vacuum level at the surface. This condition is favorable for electron emission from the photocathode surface because electrons excited in the bulk can diffuse toward the surface where they can tunnel or be ballistically emitted from the crystal into the vacuum.
The problem is that the Cs coating step can only be performed in a vacuum, because Cs is highly reactive with oxygen and therefore unstable in oxygen containing environments. The surface activation of the photocathode is therefore extremely difficult and expensive to perform, and the “cesiated” device is neither robust nor permanent. It is unstable and not long lasting, being subject to attack when exposed even slightly to oxygen atoms or molecules. On the other hand, Cesium coating of the GaAs surface provides highly desirable photocathode attributes, specifically (a) a high yield of photoelectrons when under illumination (because it has a short photon absorption length for efficient photon absorption, a long electron diffusion length to minimize photoelectron losses, and a small or negative electron affinity), and (b) high conductivity to avoid charging due to electron loss by photoemission. It has seemed that such desirable attributes could only be realized through the expensive and impermanent Cesium coating step. The result is that the photocathode is chemically unstable. What is needed, therefore, is another way of activating the photocathode that realizes all of the foregoing desirable attributes (high yield and high conductivity) while at the same time providing a structure that is highly stable and robust both chemically and physically (unlike cesiated GaAs structures).
In addition, it would be desirable to have a material system that is activated for photoelectron emission as mentioned above in which the photon absorption spectrum can be independently adjusted without compromising or sacrificing any of the foregoing desired attributes. In particular, for some space applications it is important that the sensor be blind to visible light (e.g., from the sun), and therefore it is useful to be able to set the photon absorption spectrum of the detector to exclude certain wavelength regions while including desired ones.